[seanwitte]

I haven't seen anyone take the Wallace approach and solve this with technology. As an example, you could also use retro-reflective IR sensors to look for the lines and keep track of your proximity to them. You can mount them off-vertical to give you the ability to "see" the line from a few inches away. With a little programming magic I can think of a few novel applications:

1) Flash an OI LED when you've completely crossed a line and are still within sensing distance of it. The flashing LED means "don't drive backwards!"
2) Write a dashboard app to bark at the driver when there is a danger of violating the <G22> rule.
3) Have a little stop sign pop up on the robot when you're over the line, then pop down when you've crossed. When you see the sign go down you know not to back up.
4) Prevent the robot from moving if the movement will cause a <G22> penalty

Adding one sensor to each corner of the robot would be sufficient. Assuming you're normally driving around in the forward direction you can derive the state model for the sensors.